This invention relates to a method of operating an alkali chloride electrolytic cell. More particularly, it relates to a method of operating an alkali chloride electrolytic cell which maintains a high current efficiency while preventing deterioration of the ion exchange membrane.
A method of obtaining a caustic alkali by electrolysis of an alkali chloride aqueous solution by an ion-exchange membrane method which uses a gas diffusion cathode is known. This method is achieved with an electrolysis cell partitioned with an ion-exchange membrane, usually a cation-exchange membrane, into an anode chamber having an anode and containing an alkali chloride aqueous solution and a cathode chamber having a cathode and containing water or a caustic alkali aqueous solution, in which a gas diffusion cathode made of a porous body and supplied with an oxygen-containing gas is used as the cathode to produce a caustic alkali in the cathode chamber. The merit of this technique consists in that hydrogen gas does not generate on the cathode so that the requisite electrolytic voltage is markedly reduced.
Documents disclosing the above-mentioned electrolysis method includes JP-A-54-97600 (The term xe2x80x9cJP-Axe2x80x9d used he rein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-56-44784, JP-A-56-130482, JP-A-57-152479, JP-A-59-133386, JP-A-61-266591, JP-B-58-44156 (The term xe2x80x9cJP-B-xe2x80x9d used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d), JP-B-58-49639, JP-B-60-9595, and JP-B-6120634.
As is recognized from the teachings disclosed, the subjects of conventional studies on electrolysis by the ion-exchange membrane method have been confined to the techniques for producing, or improving of the performance of, a gas diffusion cathode, with little consideration given to the improvement on the method of operating the electrolytic cell used in the ion-exchange membrane method. In order to efficiently operate the electrolytic cell of this type, it is important to establish such operating conditions as to keep the ion-exchange membrane in its best condition.
In carrying out alkali chloride electrolysis by a conventional ion-exchange membrane method without using a gas diffusion cathode, the cell is partitioned with an ion-exchange membrane into an anode chamber having an anode and a cathode chamber having a cathode, and an alkali chloride aqueous solution is fed to the anode chamber, where chlorine gas is produced; and water or a dilute aqueous solution of a caustic alkali is fed to the cathode chamber, where a caustic alkali and hydrogen gas are produced. The anode and the cathode are made of gas- and liquid-permeable materials. An anolyte (aqueous alkali chloride solution) and a catholyte (aqueous caustic alkali solution) are supplied from the back side of the respective electrodes, and the gas generated on the electrodes is discharged to the back side of the respective electrodes. The cell being so constructed, the anode chamber and the cathode chamber contain gas and liquid in a mixed state so that the gas produces a liquid agitating effect. As a result, the alkali chloride concentration in the anode chamber and the caustic alkali concentration in the cathode chamber are both maintained substantially uniform.
The situation in the alkali chloride electrolytic cell having a gas diffusion cathode is different. That is, an anode chamber having an anode, an ion-exchange membrane, a caustic chamber, a gas diffusion cathode, and a gas chamber are arranged in this order, and an alkali chloride aqueous solution is fed to the anode chamber, where chlorine gas evolves. In this respect, the cell structure is basically the same with that used in the method using no gas diffusion electrode. The difference lies in that an oxygen-containing gas is fed to the gas chamber which is adjacent to the gas diffusion cathode on the side opposite to the ion-exchange membrane and that a caustic alkali is produced in the caustic chamber provided between the ion-exchange membrane and the gas diffusion cathode. The caustic chamber is supplied with water or a dilute aqueous solution of a caustic alkali.
Because no gas evolves on the gas diffusion cathode, there is no gas in the aqueous caustic alkali in the caustic chamber. Accordingly, the aqueous caustic alkali is not to be agitated by generated gas as is observed with the case of electrolysis using no gas diffusion cathode. It follows that the caustic alkali concentration in the caustic chamber tends to be non-uniform. In order to avoid this, circulation of a caustic alkali aqueous solution has been adopted.
Circulation of a caustic alkali aqueous solution has also been used in conventional electrolysis systems having no gas diffusion cathode. In these systems, the discharged caustic alkali concentration ranges from 30 to 35% by weight, while the concentration of the supplied dilute caustic alkali is lower than that by a few percents.
Where the circulation system is applied to the alkali chloride electrolytic cell having a gas diffusion cathode, the caustic alkali concentration distribution in the caustic chamber can be controlled within several percents, which has been accepted non-problematical in running. However, when such operation is continued for a long period of time, a reduction in current efficiency can result because of deterioration of the ion-exchange membrane. The cause of the deterioration of the ion-exchange membrane is assumed ascribable to the substantial increase in caustic alkali concentration in the vicinities of the ion-exchange membrane.
It is therefore an object of the present invention to provide a method of operating an alkali chloride electrolytic cell having a gas diffusion cathode to electrolyze an alkali chloride aqueous solution to produce chlorine and a caustic alkali, in which the ion-exchange membrane is prevented from deterioration thereby to maintain high current efficiency for an extended period of time.
Other objects and effects of the present invention will become apparent from the following description.
The present inventors have extensively investigated a countermeasure against deterioration of the ion-exchange membrane in the production of chlorine and a caustic alkali by electrolyzing an alkali chloride aqueous solution in an electrolytic cell having a gas diffusion cathode. As a result, they have found that the object is accomplished by feeding a caustic alkali aqueous solution to the caustic chamber between the cation-exchange membrane and the gas diffusion cathode at or above a given flow rate. They have found that the caustic alkali concentration in the vicinities of the ion-exchange membrane is maintained on a proper level by controlling the flow of the aqueous caustic alkali solution in the caustic chamber thereby to prevent the ion-exchange membrane from being deteriorated and, as a result, high current efficiency can be retained for a prolonged period of time.
The present invention provides the following methods of operating an alkali chloride electrolytic cell.
(1) A method of operating an alkali chloride electrolytic cell, comprising:
providing an alkali chloride electrolytic cell comprising a gas diffusion cathode and a cation-exchange membrane, said gas diffusion cathode and cation-exchange membrane defining a caustic chamber;
allowing a caustic alkali aqueous solution to flow in said caustic chamber at a linear velocity of at least 1 cm/sec.
(2) The method according to the above method (1), wherein the linear velocity of said caustic alkali aqueous solution is from 1 cm/sec to 10 cm/sec.